Quo vadis, thunk? (Was: Re: [PATCH/RFA] PR target/16665: SH thunk

[Joern Rennecke]

> 
> Joern Rennecke wrote:
> 
> >I see that you wrote the comment in cp/method.c .  Is target port supposed
> >to be able to rely on the thunk being emitted together with the function
> >that is being thunked to?
> >  
> >
> Yes, where "together with" means "in the same object file".  That much 
> is required by the C++ ABI.  The ABI does not seem to say whether or not 
> the thunks must be COMDAT, but it would make sense to make them COMDAT 
> only when their associated function is also COMDAT, and in that case 
> they should all be in the same group.
> 
> Thus, I would consider the fact that G++ presently makes the thunks weak 
> even when the underlying function is not weak to be an odd quirk, if not 
> a bug.  I think that could safely be changed without endangering 
> anything.  It may be that it is the way it is for historical reasons; it 
> could also be that until my recent linkage cleanup, one could not be 
> sure whether the associated function would be COMDAT or not.  In any 
> case, I think you could change it now.
> 
> In the case that the associated function is weak, you cannot (since GCC 
> does not really do COMDAT groups) assume that the thunks will be "near" 
> the associated function; they will still be in separate .gnu.linkonce 
> sections.

No, there is code in use_thunk that sets the section of the thunk to match
the section of the function if the function is DECL_ONE_ONLY:

  if (TARGET_USE_LOCAL_THUNK_ALIAS_P (function)
      && targetm.have_named_sections)
    {
      resolve_unique_section (function, 0, flag_function_sections);

      if (DECL_SECTION_NAME (function) != NULL && DECL_ONE_ONLY (function))
        {
          resolve_unique_section (thunk_fndecl, 0, flag_function_sections);

          /* Output the thunk into the same section as function.  */
          DECL_SECTION_NAME (thunk_fndecl) = DECL_SECTION_NAME (function);
        }
    }

AFAICS after the first resolve_unique_section call
DECL_SECTION_NAME (function) should always be set if the function is
DECL_ONE_ONLY and the port fully supports named sections.